In-situ monitoring and mapping of electrical activity of devices play an important role in test and fault analysis of integrated circuits (ICs). As front-side access for probing is practically limited due to the increased metallization and packaging, back-side laser-beam techniques have been widely used to investigate transient changes of charge density in the vicinity of the devices [1, 3, 5]. The presence of free-carriers alters the local refractive index and thus influences the intensity of the focused laser beam reflected from the device. However, the amplitude of the reflectance modulation is typically on the order of parts per million of the reflected beam, rendering it a challenge to perform sensitive and quantitative measurements [9]. Further, the technical complications associated with advancing manufacturing technology such as increasing substrate doping levels and decreasing feature size and supply voltage will necessitate even higher detection sensitivity and signal localization capability.